I've sent wood to Canada several times with no problems. I've always used the on-line International Priority click n ship because it's so convenient & the customs forms are filled out automagically.
I don't know how it compares to First Class postage, but you can ship up to 4 pounds for $20.85 using International Priority - just don't go a single ounce over that amount or the price jumps to $41.37.
International Priority is the same price for a small flat rate box, a regular priority mail envelope or a padded envelope. So, I generally pack blanks in a small flat rate box, then put that inside a padded envelope & if I have some weight allowance left, I'll cram a few extra blanks around the box to fill the envelope as full as I can & stay within the 4 pound limit.
USPS has a clear stick-on sleeve with a slit that you are supposed to put the shipping label/customs form into so it's easy for the inspectors to remove then replace when they are done. I always make 2 copies of the label and tape one to the bottom of the package (fully covered with clear strapping tape) just in case the inspector loses or damages the one in the sleeve.
As noted by others, be sure there is no bark and the wood is nice & solid so there won't be any bug concerns.